This invention relates to a frequency dividing circuit for dividing an input signal of an input frequency into an output signal of an output frequency, more particularly, to a frequency dividing circuit which is capable of varying a dividing ratio.
A frequency dividing circuit of the type described comprises a first and a second delay flip flop circuit. The first delay type flip flop circuit is connected to the second delay type flip flop circuit in cascade connection. Responsive to an input signal of an input frequency, the first delay flip flop circuit produces a first delay signal and an inverted delay signal which has an antiphase relative to the first delay signal. Responsive to the first delay signal, the second delay type flip flop circuit produces a second delay signal which is outputted as an output signal of an output frequency.
In a conventional frequency dividing circuit, a mode signal, which is representative of a timing for changing a dividing ratio, is used in order to vary the dividing ratio. The mode signal is supplied together with the second delay signal to a NOR gate. The NOR gate produces a NOR gate signal in response to the mode signal and the second delay signal. The NOR gate signal is supplied together with the inverted delay signal to an OR gate. The OR gate produces an OR gate signal in response to the NOR gate signal and the inverted delay signal to supply the first delay type flip flop circuit with the OR gate signal as an original signal.
As described above, the second delay signal is supplied as the original signal to the first delay type flip flop circuit through the NOR gate and the OR gate. Therefore, it is difficult to operate at a high speed in the conventional frequency dividing circuit because of a delay time in the NOR gate and the OR gate.